It is well known that the particle size of pigments used by the paper industry has a great effect on functional properties. It has long been recognized that in cases of kaolin clay filling pigments, the greater the amount of particles finer than 2 micrometers (e.s.d.) in the clay, the less opaque is the sheet. This has been explained by the fact that the kaolin particles finer than 2 micrometers have a different morphology from the particles greater than 2 micrometers. The 2 micrometers particle size is the approximate cutoff point below which the kaolinite particles in a naturally occurring kaolin may exist as thin individual crystalline plates. Above this cutoff point the kaolinite particles exist as bonded stacked aggregates of platelet (so called "stacks" or "booklets") or, in some cases, long, wormlike assemblies of platelet, which make the kaolin more bulky, thus leading to better opacifying power.
Conventional kaolin clays crudes used as a sources of pigment grades of kaolin usually contain about 40% to 60% by weight of particles finer than 2 micrometers after removal of grit and coarse impurities. The kaolin portion of the crude is polydispersed in the sense that the particles occur over a range of sizes and shapes. Thus, a kaolin crude will not contain particles of a single size, such as, for example, particles all of which are 2 micrometers. Typically, a degritted kaolin crude will contain particles ranging in size from sub-micron (or colloidal) to particles that are 20 micrometers or larger. Such degritted clays are too coarse to be used in fine paper making. Conventional practice is to remove a preselected portion of plus 2 micrometers from the finer clay particles. This is accomplished by forming the crude clay into a dispersed aqueous pulp, removing the gross impurities by means such as screening, and then hydraulically fractionating the clay to remove a desired proportion of clay particles larger than 2 micrometers. The fractionation is usually carried out by using centrifuges in commercial operations. The product from the centrifuge is a suspension having a higher percentage of finer particles than the starting clay. After bleaching (and possibly other beneficiation), the fine fraction may be sold as a paper coating pigment. When the centrifugation is registered trademark "NOKARB", are used to a limited extent as fillers. Frequently a portion of, or even the entire, coarse fraction is discarded because of the limited demand for such clays.
In recent years, some of the coarse particle size fractions of kaolin crudes have been used as feed material in the production of mechanically delaminated kaolin pigments. It has long been recognized in the industrial minerals industry that mechanical delamination of kaolin stacks present in the coarse particle size fraction of many kaolin crudes provide kaolin filler products with improved opacification in filling applications. See, for example U.S. Pat. No. 3,171,718, Gunn et al. Commercially available delaminated No. 2 grades used as fillers by the paper industry are supplied commercially under the registered trademarks NUFIL and NUFIL 90.
In practicing delamination, a suspension of a coarse particle size fraction of a kaolin crude is agitated with grinding media such as sand, plastic pellets or glass microballons until the booklets in the feed clay are cleaved and delaminated particles are produced. Similar results may be obtained when a paste of clay is extruded under high pressure ("superstrusion"). After the kaolinite booklets are delaminated, a suspension of the clay may be fractionated. A fine size fraction containing the artificially produced platelet with dimensions of 2 micrometers and finer is recovered. The fine size fraction is used as a paper coating pigment.
Particle sizes of kaolin are conventionally determined by sedimentation using Stoke's Law to convert settling rates to particle size distribution, and assume a spherical particle shape for the kaolin particles. Hence, the use of conventional term "equivalent spherical diameter (e.s.d)" to designate particle size. It is well known that the expression of the particle size in terms of equivalent spherical diameter is not accurate in the case of particles having a high aspect ratio such as mechanically delaminated platelets.
It has also been recognized that a relative narrowing of the particle size distribution of mechanically delaminated as well as non-delaminated kaolin particles results in pigments providing improved opacity in filling applications. Such pigments are disclosed as being especially advantageous when used in the manufacture of lightweight coated paper for rotogravure printing (see U.S. Pat. No. 4,948,664, Brociner et al). It is well known to remove ultrafine kaolin particle, e.g., particles finer than about 0.3 micrometers, e.s.d. after delamination. This obviously contributes to the production of a delaminated pigment product having a narrower particle size distribution than it would have if the ultrafines were not removed. U.S. Pat. No. 4,948,664, supra, shows that in cases where very narrow particle size distribution was required, delamination was followed by a coarse fractionation and secondary fine removal steps. Patentees did not remove the primary fines prior to delamination. In illustrative examples, there was a significant amount of secondary fines that were generated during delamination which had to be removed latter. Removal of fines is referred to as "defining" in U.S. Pat. No. 4,943,324 Bundy et al., and U.S. Pat. No. 5,085,707, also Bundy et al. In these patents, at least a portion of slimes is present dduring delamination. Sometimes removal of the fines is termed "desliming".
Typical commercial delaminated kaolin pigments, exemplified by NUCLAY (or NUFIL) kaolin supplied by Engelhard Corporation, are about 80% finer by weight finer than 2 micrometers, e.s.d. Thus, the particle size distribution of such commercial delaminated kaolin as measured by sedimentation is typical of that of No. 2 non-delaminated coating clay. U.S. Pat. No. 5,169,443 Willis et al., commonly assigned, makes reference to certain delaminated kaolins that are about 90% by weight finer than 2 micrometers, thus similar to No. 1 coating clays.
Surface area is a property related to particle size of kaolins although surface area alone does not correlate directly with particle size. Surface area is expressed in terms of square meters of area per gram of a material and is frequently measured by the BET method using nitrogen as adsorbate. MerCury porosimetry is also used to measure surface area. A No. 1 clay, which typically has a large number of fine particles, has higher surface area than a No. 2 clay, because fine particles have a higher surface area than coarse particles. Use of surface area as a parameter in evaluating delaminated kaolins appears in U.S. Pat. No. 5,169,443, Willis et al., supra.
Filler pigments of the invention are useful in making various grades of paper. However, their outstanding performance is especially noteworthy in filling those grades referred to as "fine" or, alternatively, as "wood free". Fine paper is a general classification referring to white, uncoated printing and writing grades which contain no more than 25% mechanical pulp in the furnish. Most furnishes are wood free. Specific grades are offset; tablet, envelope, bond, ledger, mimeo, duplication and various book stocks. The pulps used to make such grades are chemically treated to achieve higher brightness and strength than papers such as newsprint prepared from ground wood pulp.
Generally, wood-free paper is 20-25 points brighter than ground wood paper. However, because of the higher brightness of the wood free paper, opacity is much lower than ground wood paper.
Fine paper is made in a wide range of basis weight from 36 pounds to 180 pounds/3,300 ft..sup.2, depending on the intended application. Further, uncoated fine paper, such as Xerox.RTM. paper, gives a significantly better printablility as compared to paper from ground wood pulp because the sheet is tighter and there is less absorption of ink in the base stock.
Expensive pigments such as calcined kaolin pigments and titania are normally used to increase the opacity of filled sheets of fine papers. Pigments of this invention are used to extend these more expensive pigments, thus reducing pigment cost. In this regard, it is noted that unfilled wood free paper is expensive. Pigments of this invention will replace fiber, thus saving paper mills money in addition to money savings attributed to extension of more expensive pigments such as calcined kaolin and titania.